MRI (magnetic resonance imaging) systems are used for medical diagnosis. A requirement of an MRI magnet is that it provides a stable, homogeneous, magnetic field. In order to achieve stability it is common to use a superconducting magnet system which operates at very low temperature, the temperature being maintained by cooling the superconductor, typically by immersion in a low temperature cryogenic fluid, such as liquid helium, liquid neon, liquid hydrogen or liquid nitrogen. Cryogenic fluids are expensive, and it is desirable that the magnet system should be designed and operated in a manner to reduce to a minimum the amount of cryogenic fluid used.
The superconducting magnet system typically comprises a set of superconductor windings for producing a magnetic field, contained within a cryostat. The cryostat typically comprises a cryogenic fluid vessel which contains the superconductor windings, one or more thermal shields completely surrounding the cryogenic fluid vessel, and a vacuum jacket completely enclosing the one or more thermal shields. It is common practice to use a refrigerator to cool the thermal shields to a low temperature in order to reduce the heat load onto the cryogen vessel. It is also known to use a refrigerator to directly refrigerate the cryogen vessel, thereby reducing or eliminating the cryogen fluid consumption.
MRI magnet systems use refrigerators to reduce the heat load onto the cryogen vessel in order to reduce or eliminate the consumption of cryogenic fluid. The refrigerator must make good thermal contact to the objects to be cooled whilst being easy to remove and replace for servicing.
The refrigerator is subject to wear, and must be periodically serviced and then replaced after a certain time in order to maintain adequate system performance. The refrigerator is therefore placed in a means of removably interfacing it to the magnet system.
Achieving good thermal contact at low temperature is difficult, and whilst adequate thermal contact can be achieved using pressed contacts at the thermal shield temperatures, these pressed contacts are difficult to re-make once the refrigerator has been removed and replaced, for example as required for servicing. The thermal contact provided needs to allow such removal and replacement without significant degradation in the thermal transfer efficiency of the thermal contact, otherwise it becomes more difficult to achieve the desired refrigeration at very low temperature.
Known solutions are discussed, for example, in U.S. Pat. Nos. 5,782,095, 5,613,367, 5,918,470; and in European Patent Application EP 0 720 024 A1.
In each of these described solutions, a cryogenic refrigerator is provided, removably mounted within a sleeve. In each case, the sleeve terminates in a heat sink which is thermally linked in some way to a recondensing surface exposed to a cryogen vessel. In U.S. Pat. No. 5,613,367, the recondensing surface 32 is directly exposed to the interior of the cryogen vessel. In each of the remaining solutions mentioned above, a separate recondensing chamber is provided, adjacent to the heat sink. Gas supply and liquid return pipes are provided to link the recondensing chamber with the cryogen vessel. In each of these solutions, there exists a requirement to provide an effective thermal interface between the refrigerator and the heat sink. In U.S. Pat. No. 5,918,470 and EP 0 720 024, this is provided by a compressible indium washer. In U.S. Pat. No. 5,613,367, the corresponding thermal interface relies upon effective cleaning of mechanically touching parts.
Problems are encountered when removing and replacing the refrigerator, in that the thermal coupling between the refrigerator and the heat sink may degrade, or at least is difficult to maintain. When the refrigerator is removed from its sleeve, the interior of the sleeve is exposed to the atmosphere. The heat exchangers will be at very low temperatures, and it is difficult to prevent atmospheric gases from condensing or freezing on the inside of the sleeve.
Such condensate may be removed by bringing the temperature of the sleeve to ambient, but this will result in a more lengthy process for removal and replacement of the refrigerator, and may lead to unwanted cryogen loss. Furthermore, the use of indium washers requires that a new indium washer be used every time that the refrigerator is removed and replaced. Indium washers suffer from creep—that is, the indium washer providing the thermal contact between the refrigerator and the heat exchanger changes shape over time, resulting in degradation of the thermal interface. In addition, when the refrigerator is removed for servicing, it is difficult to remove all traces of the old indium washer before fitting a new one. Any residue of the earlier washer will cause non-uniform compression of the later washer, resulting in a sub-standard thermal interface.